It is widely known that honeycomb structures formed from ceramic or other porous materials can be used to filter solid particulates and larger particles from fluids passed therethrough including carbonaceous particulates from the exhaust gases of diesel engines. There are two basic types of honeycomb solid particulate honeycomb filters: unidirectional flow and cross-flow. This invention relates to the former.
A unidirectional flow type honeycomb filter is formed by a matrix of thin interconnected porous walls defining an inlet and an outlet end face on opposing outer surfaces of the filter and a multiplicity of hollow cells extending through the filter between the two end faces. An inlet group of cells is formed by closing the open ends of some of the cells near the outlet end face. An outlet group is similarly formed by closing the open ends of other cells near the inlet end face. A porous outer wall is typically provided around the matrix and between the end faces. Fluid enters the filter primarily at the inlet end face through the inlet cells but may also enter through the outer wall, if porous. The thin walls of the matrix are provided with internal interconnected open porosity of a volume and size sufficient to enable the fluid to flow at least across their narrow dimensions and, if desired, through their longer dimensions between adjoining and/or neighboring cells while preventing at least a significant portion of the particulates and larger particles from flowing in any direction across and through the thin walls. Trapped particulates are deposited on and within the thin wall surfaces forming the inlet cells. Diesel exhaust particulate filters (hereinafter referred to as "DPF's"), as well as molten metal and heat recovery wheel filters of the unidirectional flow type are described in a pending application Ser. No. 165,646, filed July 3, 1980 and assigned to the assignee hereof, which is incorporated by reference herein.
Unidirectional flow type honeycomb filters are preferred for diesel particulate filter application because they are relatively straightforward to manufacture and can be mounted in a housing and inserted into an exhaust system like a muffler or catalytic converter. Prior practice has been to maximize the cross-sectional diameter of a DPF transverse to its cells to the extent allowed by vertical and lateral vehicular clearances and then to extend the length of the filter to provide the volume required to accomplish the desired filtration of the exhaust gases. This has typically resulted in DPF's having lengths greater, often many times greater, than their diameters.
It is desirable to maximize the useful operating time and/or particulate loading capacity of DPF's to minimize the cost and inconvenience associated with their replacement and/or regeneration. Both characteristics are effectively limited, among other factors, by the back pressure generated or flow rate allowed by the filter. A filter has an initial pressure drop (i.e., the difference in pressure between the contaminated fluid upstream and filter fluid downstream caused by the presence of the filter therebetween) which increases during use with the entrapment of particles and particulates in and on the thin walls forming the filter's inlet cells. Similarly the filter also has an initial flow rate which decreases with particulate build-up. Depending upon the application, either may control when the filter must be replaced or regenerated. The useful operating life of a DPF is generally controlled by the maximum back pressure which can be sustained by the diesel engine with which it is used.